As such a separation prevention device for a pipe joint, there is known a separation prevention device including disengagement prevention members disposed at a plurality of locations in a circumferential direction of the pipe on an inner peripheral surface of an engaging tool which engages with a receiving opening or an inner peripheral surface of the receiving opening itself, and a press member which presses the disengagement prevention members inward in the radial direction of the pipe, as disclosed in Patent Documents 1 and 2. A claw is formed on the inner peripheral surface of the disengagement prevention member. The disengagement prevention member is pressed by the press member, the claw is made to bite into an outer peripheral surface of the inserting opening and engage with the outer peripheral surface, thereby exhibiting a separation preventing effect against a separating force of the receiving opening and the inserting opening in the axial direction of the pipe.
One example of the conventional separation prevention device of a pipe joint will be described with reference to FIGS. 11 to 15. The same reference numerals are attached to the same members and positions as the members and positions described in a first embodiment according to the present invention mentioned below, and an overlapping description will not be repeated. As shown in FIGS. 11 to 13, a pressing ring 14 is engaged with a receiving opening 1 in a state where the pressing ring 14 is fitted over an inserting opening 2, the pressing ring 14 presses a seal material 3 inserted into the receiving opening 1 and compresses the seal material 3 to a sealed state. FIG. 12 is a cross sectional view as seen from an arrow F-F in FIG. 11. FIG. 13 is a cross sectional view as seen from an arrow G-G in FIG. 11
FIG. 14 is a development view of the inner peripheral surface of the pressing ring 14 as viewed from the direction of an arrow H in FIG. 11. Recesses 19 are formed at a plurality of locations in an inner peripheral surface 14a of the pressing ring 14 at equal distances from one another in the circumferential direction of the pipe. The recesses 19 open toward the inserting opening 2. A disengagement prevention member 16 having a claw 18 is accommodated in each of the recesses 19. A screw hole is formed in each of the recesses 19 radially outward of the pipe, and a pressing bolt 17 as the press member is threadedly engaged with the screw hole. The separation preventing effect of the pipe joint is exhibited by operating the pressing bolt 17 to press the disengagement prevention member 16 radially inward of the pipe, and by making the claws 18 bite into and engage with the outer peripheral surface of the inserting opening 2.
According to the separation prevention device, however, it was found that when the claws 18 of the disengagement prevention members 16 were made to bite into and engage with the outer peripheral surface of the inserting opening 2 as shown in FIG. 15, in a material limit region where a pipe is deformed, stress concentration occurred due to pipe deformation in a non-contact region Y formed between contact regions X of the claw, and a crack was generated in a lining layer M of the inner peripheral surface of the pipe in some cases. The crack is frequently generated especially at a central portion of the non-contact region Y. It is conceived that this is because a shearing stress generated near ends of the claws 18 which are adjacent to each other in the circumferential direction of the pipe acts on the lining layer M at the central portion of the non-contact region Y.
In the separation prevention device in which the disengagement prevention members are disposed at the plurality of locations in the circumferential direction of the pipe, since a ratio of a length of the contact region of the claw in the circumferential direction of the pipe with respect to the outer peripheral surface of the inserting opening is about 80 to 90% at most, in order to enhance the separation preventing effect, there is room for improvement of the rate. Particularly, there is a tendency that it is difficult to prevent the separation against strong external forces caused by an earthquake or differential settlement, and it is strongly desired to propose a technique for improving this.    Patent Document 1: Japanese Unexamined Patent Publication No. 61-88091    Patent Document 2: Japanese Unexamined Patent Publication No. 2005-3207